Application of Sodium Alginate with Noni Leaf Extract (Morinda citrifolia L.) as a Potential Preservative for Tomatoes (Solanum lycopersicum)

Introduction

Synthetic preservatives are chemicals used to prevent food spoilage or contamination by microorganisms, as well as to enhance the color and taste of food. Scientifically, a growing body of evidence suggests that the use of synthetic chemicals as food additives may have adverse effects on human health¹. In the field of food preservation, the challenge remains to identify sustainable alternatives to synthetic preservatives while balancing safety, efficacy, and environmental responsibility².

In recent years, edible coatings have emerged as an effective approach for extending the postharvest life of fresh produce. These coatings create a thin, edible matrix on fruit and vegetable surfaces that moderates water vapor transfer and suppresses microbial proliferation, thereby helping to maintain quality during storage³. Among natural biopolymers, sodium alginate, a polysaccharide derived from brown seaweed, has attracted considerable interest because of its strong film-forming capacity, safety for food applications, and environmentally friendly nature. Alginate-based coatings are extensively applied to horticultural commodities to slow respiration processes, delay ripening, and reduce postharvest deterioration⁴

Postharvest tomato fruits (Solanum lycopersicum) rank among the most commonly consumed horticultural crops globally and are highly prone to microbial spoilage, leading to significant economic losses and food waste⁵. Conventional preservation approaches often rely on synthetic additives, raising concerns regarding consumer health and environmental impact. Consequently, there is an increasing need to explore alternative preservatives that provide both antimicrobial efficacy and ecological sustainability. Natural plant-based preservatives, particularly herbs with antimicrobial properties, can inhibit bacterial and fungal growth and thereby improve food quality and extend shelf life⁶.

Morinda citrifolia L., commonly known as noni, is among the plant-based sources being explored for food preservation. Noni has long been utilized in traditional health practices for various diseases⁷. According to Mubarokah et al. (2023), ⁸ noni leaf extract contains bioactive compounds, including flavonoids, phenolics, and alkaloids, which contribute to its antimicrobial and antioxidant properties. These properties make noni leaf extract a promising natural preservative capable of inhibiting spoilage microorganisms and delaying oxidative degradation in tomatoes. The bioactive potential of noni leaf extract has been widely reported in the literature⁹, supporting its potential as a safer alternative to synthetic preservatives, which are often associated with health concerns. However, limited studies have investigated the combined application of sodium alginate and noni leaf extract in food preservation.

Therefore, the present study investigates the effectiveness of sodium alginate coatings enriched with noni leaf extract as a natural preservation strategy for postharvest tomatoes, with the aim of improving storage quality while reducing dependence on synthetic preservatives. 

Materials and Methods

Chemicals

All reagents utilized in this work were of either analytical or food-grade quality. A 70% (v/v) ethanol solution was employed as the solvent for extraction. Food-grade sodium alginate was used as the film-forming polymer, while analytical-grade calcium chloride functioned as the cross-linking agent. Glycerol of food-grade quality was incorporated as a plasticizer in the coating formulations.

Sample collection and preparation

Tomatoes (Solanum lycopersicum) were sourced from a public market in Talavera, Nueva Ecija. The fruits were selected at the ripening and turning stages, corresponding to 60–90% red surface coloration. Only tomatoes exhibiting uniform color, firmness, size, and shape, and free from physical defects or microbial decay, were included. The selected fruits were washed with distilled water to remove surface debris, then drained and air-dried at room temperature prior to pretreatment. The preparation procedure was adapted, with minor modifications, from the protocol reported by Álvarez-Pérez et al. (2025) ¹⁰.

Preparation of noni leaf powder

Fresh noni leaves were harvested from Pulong, San Miguel, Talavera, Nueva Ecija. Leaf powder preparation was carried out based on a previously reported procedure by Olatunde et al. (2020) ¹¹, with minor procedural adjustments. The leaves were thoroughly rinsed under running water and dehydrated at 50 °C until the moisture content decreased to less than 10%. The dried material was then milled into a fine powder using a laboratory blender and passed through an 80-mesh stainless steel sieve to obtain uniform particle size.

Preparation of ethanolic noni leaf extract

A measured amount (100 g) of noni leaf powder was subjected to solvent extraction using 70% (v/v) ethanol at a solvent-to-material ratio of 10:1 (v/w). The extraction was performed by maceration at ambient temperature for 72 hours. The extract was subsequently separated by filtration through Whatman filter paper to remove solid residues. The collected filtrate was concentrated under reduced pressure at 45 °C using a rotary evaporator. The concentrated extract was then dispensed into sterile containers and preserved at 4 °C until further analysis¹⁰.

Preparation of coating solutions

The coating formulations were developed based on previously reported methods¹²˒¹³ with minor procedural adjustments. Sodium alginate coatings were prepared by dispersing 1% (w/v) sodium alginate in distilled water, followed by continuous stirring at 70 °C for 1 hour until complete dissolution and clarity were achieved. The solution was allowed to cool to ambient temperature, after which glycerol (1%, v/v) was incorporated as a plasticizer and mixed for 15 minutes to obtain a homogeneous mixture. Ethanolic noni leaf extract was subsequently added to the alginate solution at concentrations of 1%, 3%, and 5% (v/v) and stirred for an additional 30 minutes to ensure uniform distribution. The compositions of the resulting coating formulations are summarized in Table 1.

Table 1: Coating Treatments for Tomatoes

Application of Sodium Alginate Coating incorporated with Noni leaves extract to Tomatoes

The tomato fruits were randomly assigned to five experimental groups. Uncoated fruits served as the control, while the remaining groups were treated with 1% sodium alginate coatings supplemented with 0% (T1), 1% (T2), 3% (T3), and 5% (T4) noni leaf extract. The fruits were submerged in the respective coating formulations for 2 minutes, then removed and allowed to dry naturally at ambient conditions. To enhance coating stability and adhesion, the coated tomatoes were subsequently immersed in a 2% (w/v) calcium chloride solution for an additional 2 min to induce polymer crosslinking¹²˒¹³. All samples were arranged on trays and stored under room temperature conditions for 14 days.

Characterization of Ethanolic Noni leaf extract

Functional group analysis of the ethanolic noni leaf extract was conducted using Attenuated Total Reflectance–Fourier Transform Infrared (ATR-FTIR) spectroscopy. A small quantity of the extract was applied directly onto the ATR crystal surface, and infrared spectra were collected across the wavenumber region of 4000–400 cm⁻¹¹⁴.

Measurement of Weight Loss

Tomato weight loss over the 14-day storage period was evaluated by measuring the mass of the fruits at the beginning and at the end of storage periods^{15, 25}. Weight loss was calculated and expressed as a percentage using the equation below:

%Weight loss = [ (initial weight – final weight) / initial weight] x 100

pH Determination

Tomato samples were homogenized and filtered through cheesecloth to obtain clear extracts. The pH of the resulting filtrates from both control and treated samples was measured using a calibrated digital pH meter¹⁶.

Total soluble solids (Brix ̊)

Total soluble solids (TSS) were quantified using a handheld refractometer, and results were reported as degrees Brix (°Brix). The refractometer prism was rinsed thoroughly with distilled water and wiped dry between successive measurements to avoid cross-contamination¹⁷.

Determination of Firmness

Fruit firmness was evaluated using a digital penetrometer (Model GY-3) fitted with an 8-mm diameter probe. Penetration tests were conducted by applying uniform pressure to the fruit surface to ensure measurement consistency. Firmness readings were recorded and expressed as kg cm⁻² ¹⁸.

Statistical analysis

All experimental data were subjected to one-way analysis of variance (ANOVA) to assess differences among treatments. When statistically significant effects were observed, Tukey’s Honestly Significant Difference (HSD) test was performed for post hoc mean comparisons at a 5% level of significance using SPSS software.

Results and Discussion

Characterization of ethanolic noni leaf extract

Fourier transform infrared (FTIR) spectroscopy is a widely used analytical technique for identifying changes in the overall biomolecular composition of samples through the detection of functional groups¹⁹.

Figure 1: FTIR spectra for Ethanolic Noni leaf extract Click here to View Figure

Table 2: Peak values and functional groups of ethanolic noni leaves extracts

 The FTIR profile of the ethanolic extract of noni leaves revealed several distinct absorption bands corresponding to functional groups typically associated with plant-derived secondary metabolites (Figure 1; Table 2). A strong and broad band detected within the 3400–3200 cm⁻¹ region is attributed to O–H stretching vibrations, suggesting the presence of hydroxyl functionalities commonly originating from alcohols and phenolic constituents. Similar broad O–H absorption features have been reported in FTIR spectra of noni extracts by Yee (2019) ²⁰.

Absorption peaks appearing in the 1500–1700 cm⁻¹ range were assigned to C=C stretching vibrations, indicating the occurrence of unsaturated carbon bonds, which corroborates earlier FTIR findings on noni plant materials (Yee, 2019) ²⁰. Furthermore, bands observed between 1000 and 1100 cm⁻¹ correspond to C–O stretching vibrations, reflecting the presence of functional groups such as esters, ethers, anthraquinones, and alcohols, as previously reported in phytochemical analyses of plant extracts²¹.

These spectral features confirm that the ethanolic extract of noni leaves contains a diverse range of bioactive compounds, including phenolic compounds, anthraquinones, and flavonoids. These compounds are known to contribute to the antioxidant and antimicrobial properties of noni leaves²².

Physiological loss in weight

Weight loss is a critical indicator of postharvest quality, as it reflects moisture loss caused mainly by respiration and transpiration. In this study, sodium alginate coatings incorporated with varying concentrations of noni leaf extract (NLE) significantly affected the weight loss of tomatoes during a 14-day storage period.

As shown in Table 3, a statistically significant difference in weight loss was observed among treatments (p < 0.05). The highest weight loss occurred in tomatoes coated with sodium alginate without NLE (T1, 12.98 ± 1.39%), followed by the uncoated control (8.90 ± 1.64%). In contrast, the lowest weight loss was recorded in tomatoes treated with 5% NLE (T4, 5.56 ± 0.08%). Weight loss decreased progressively with increasing NLE concentration, indicating improved moisture retention.

Table 3: Physico-chemical Properties of Tomatoes Treated with Different Concentrations of NLE.

Results are given as averages ± SD (n = 3). Values sharing the same superscript letter within a column do not differ significantly at P > 0.05.

Treatment concentration: T1 (0% NLE), T2 =1% NLE, T3 =3% NLE, T4=5% NLE

The superior performance of T4 may be attributed to the combined barrier effect of sodium alginate and the bioactive compounds in noni leaf extract, which reduce water vapor transmission and respiration rates. In contrast, alginate coatings without NLE may be more susceptible to fungal activity, which can disrupt the coating structure and lead to increased moisture loss²³. Similar trends were reported by Zewdie et al. (2022), ²⁴who observed reduced weight loss in tomatoes coated with plant-extract-enriched edible films.

Sodium alginate–NLE coatings effectively created a semi-permeable barrier that modified the micro-atmosphere surrounding the fruit, thereby slowing transpiration and respiration and reducing postharvest weight loss²⁵.

pH

Fruit pH is a crucial quality parameter that influences flavor, microbial stability, and enzyme activity during storage²⁶. The pH values of tomatoes treated with sodium alginate–NLE coatings are presented in the same table above.

Statistical analysis showed no significant variation in pH among treatments (p > 0.05). The pH values ranged from 4.17 (3% NLE, T3) to 4.54 (1% NLE, T2), indicating that the inclusion of noni leaf extract did not modify fruit acidity. These results suggest that sodium alginate coatings with NLE do not interfere with organic acid metabolism during storage.

The obtained pH values (4.17–4.54) fall within the optimal range for tomatoes intended for fresh consumption and storage^26,27. This indicates that the coatings preserved fruit quality without negatively affecting acidity or flavor attributes.

Figure 2: Visual appearance of tomatoes coated with sodium alginate with different concentrations of noni leaves extract at Day 0, Day 7, and Day 14 of storage at room temperature  Click here to View table

Total soluble solids

Total soluble solids (TSS), reported as degrees Brix (°Brix), are commonly used to assess fruit ripening and sugar accumulation. The TSS values of tomatoes during storage are also summarized in Table 3.

No significant differences in TSS were detected among treatments (p > 0.05). Mean TSS values ranged from 3.67 ± 0.58 in T3 (3% NLE) to 4.33 ± 0.58 in the control, T1, and T4. These findings indicate that the addition of noni leaf extract, regardless of concentration, did not significantly influence sugar content during the storage period.

Although sodium alginate–NLE coatings effectively reduced weight loss and maintained firmness, they did not alter the enzymatic conversion of complex carbohydrates into simple sugars. Similar observations were reported by Manozzi et al. (2016), ²⁸who noted that edible coatings may preserve internal fruit quality without significantly affecting TSS, particularly during short-term storage.

Fruit Firmness 

Fruit firmness is a critical quality attribute influencing consumer acceptance, storability, resistance to mechanical damage, and susceptibility to decay. It also reflects underlying metabolic activity and water status within the fruit tissue. Maintaining adequate firmness is therefore essential to prolong shelf life and sustain postharvest quality²⁹.

As shown in Table 3, tomato firmness differed significantly among treatments during the storage period (p < 0.05). Firmness values varied according to the concentration of noni leaf extract (NLE) incorporated into the sodium alginate coatings. The uncoated control exhibited lower firmness compared with all coated treatments. Tomatoes treated with higher NLE concentrations (3% and 5%) maintained markedly higher firmness levels than the control (p < 0.05). Although firmness increased with increasing NLE concentration, no statistically significant difference was detected between the 3% (T3) and 5% (T4) NLE treatments (p > 0.05). Similarly, no significant differences were observed among the control, 0% NLE (T1), and 1% NLE (T2) treatments (p > 0.05).

Fruit softening during storage is primarily associated with biochemical changes in the cell wall, particularly the enzymatic degradation of structural polysaccharides such as pectin, cellulose, and hemicellulose. These processes intensify during ripening due to increased respiration and transpiration, resulting in the loss of structural integrity and firmness²⁵. In untreated fruits, these metabolic activities progress more rapidly, resulting in accelerated softening.

In this study, sodium alginate–NLE coatings, particularly those containing higher NLE concentrations, effectively delayed the enzymatic breakdown of cell wall components, thereby maintaining fruit firmness. The reduced firmness observed in the control and 0% NLE treatments can be attributed to enhanced respiration, moisture loss, and microbial activity, which promote polysaccharide hydrolysis in unprotected fruits¹⁵. The presence of noni leaf extract likely enhanced coating performance through its antioxidant and antimicrobial properties, further contributing to delayed softening.

Firmness and weight loss are closely related indicators of tomato shelf life and overall quality. The 5% NLE treatment (T4) exhibited both the lowest weight loss (5.56 ± 0.08%) and the highest firmness values, significantly outperforming the control and lower NLE treatments. This relationship suggests that increased NLE concentration improved moisture retention while simultaneously preserving textural integrity throughout storage.

Overall, the application of sodium alginate coatings enriched with noni leaf extract effectively maintained tomato firmness by reducing water migration and slowing physiological and biochemical deterioration. Additionally, these coatings may alter the internal gaseous environment of the fruit by restricting oxygen diffusion and increasing carbon dioxide levels, thereby reducing respiration rates and delaying texture deterioration¹². These findings further support the potential of sodium alginate–NLE coatings as an effective postharvest preservation strategy for tomatoes.).

Conclusion

Sodium alginate coatings incorporated with noni (Morinda citrifolia L.) leaf extract effectively preserved the post-harvest quality of tomatoes. The 5% noni leaf extract treatment showed the greatest reduction in weight loss and the highest retention of fruit firmness during storage. No significant differences were observed among treatments in terms of fruit pH and total soluble solids, indicating that the coating did not alter the intrinsic physicochemical properties of the tomatoes. These results demonstrate that sodium alginate enriched with noni leaf extract is a promising natural, biodegradable, and safe method for preserving tomatoes, extending their shelf life, and reducing post-harvest losses, with potential applications in sustainable fresh produce preservation.

Acknowledgements

The authors gratefully acknowledge the Department of Natural and Applied Sciences, Nueva Ecija University of Science and Technology (NEUST) for providing the necessary resources and opportunities to conduct this research. The authors also extend their sincere appreciation to the Philippine Carabao Center (PCC) for their assistance and for generously allowing the use of their laboratory facilities. The support of all individuals who contributed to the completion of this study is also duly acknowledged.

Funding Sources

The authors confirm that this study was conducted without financial support from any external funding agencies.

Conflict of Interest

The authors declare that there are no conflicts of interest associated with this work.

Data Availability Statement

This statement does not apply to this article.

Ethicsl Statement

This research did not involve human subjects, animal experimentation, or the use of materials requiring ethical clearance.

Author Contributions

Ma. Virgini M. Cuevas contributed to the conceptualization of the study, experimental design, execution of the experiment, data acquisition, and manuscript preparation. Ryan V. Cabanatan provided supervision, guidance, manuscript review, and editing.

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